Fibre optic communication is and will be for a long time an essential, integral component of telecommunication systems. For example, telephone networks, the Internet, cable television networks and banking networks routinely use optical fibres to transmit huge quantities of data.
The conveyed data are often of confidential nature, for example a credit card number or a password providing remote access to a computer system. To ensure confidentiality, the conveyed information is ciphered by means of “classical” methods whose security stands from the hypothesis, until now unproven but also not rebutted, that the computational time required to break the cipher is much too long [1]. However, the unforeseeable nature of scientific discoveries as well as the development of the presently known technologies may lead to techniques capable of breaking these classical ciphering methods through a quantum computer [2].
A solution to this problem was discovered in 1984 as reported in an article by C. H. Bennett and G. Brassard [3]. This solution consists of a new method called quantum cryptography (QC) or, more specifically, quantum key distribution. In the foregoing disclosure, we shall make no difference between these two expressions. Since its discovery, feasibility of quantum cryptography with optical fibre signals has been demonstrated many times (see [4] for a review of this domain). The advantage of the quantum cryptography method is that its security is unconditionally warranted by the laws of physics against any possible attack, which overcomes the problems inherent to the above-discussed “classical” ciphering methods. The presently available quantum cryptography concepts are generally designed to allow only two users to communicate with each other under absolute confidentiality.
In view of allowing a greater number of users to use quantum cryptography in an optical network, a new architecture is required.